I had a very strong mental map of the places that I was writing about in my novel. It was set in a part of Cornwall that I know well, though I also dragged in locations from other counties where I dwelled for a while - this made me feel like Zeus!
When I first started describing places I gave them rather generic names, hoping that the story would feel like it could have taken place anywhere. But then I started to get lost, and realised too that I wasn't using one of the strengths of my tale - it's distinctive landscape. I took the liberty of altering place names, so Liskeard became Liskerrett, but this is the sort of thing that readers love to work out.
I was vaguely aware of future marketing opportunities and tourist trails that could spring up - just think what Doc Martin, Daphne du Maurier and Winston Graham's Poldark have done for Cornwall - estate agents love it!
I like maps in novels, though they tend to be for historic or fantasy worlds, don't they?
I'm going off topic slightly here, but another feature of books that I appreciate and which isn't seen so much these days, is a cast of characters in the front of a complicated story.